A spatial survey of the path of a borehole is usually derived from a series of values of an azimuth angle and an inclination angle. Measurements from which values of these two angles can be derived are made at successive stations along the path, the distances between adjacent stations being accurately known.
In a borehole where the earth's magnetic field is unchanged by the presence of the borehole itself, measurements of the components of the earth's gravitational and magnetic fields in the direction of the case-fixed axes can be used to obtain values for the azimuth angle and the inclination angle, the azimuth angle being measured with respect to an earth-fixed magnetic reference, for example magnetic north. However, in situations where the earth's magnetic field is modified by the local conditions in a borehole, for example when the borehole is cased with a steel lining, magnetic measurements can no longer be used to determine an azimuth angle relative to an earth-fixed reference. In these circumstances, it is necessary to use a gyroscopic instrument.
It has already been proposed to use a gyroscopic compass in which the spin axis is set up along an earth-fixed reference line at the mouth of the borehole and, so far as possible, held fixed in inertial space. However, this procedure has many disadvantages, largely due to the necessity of constructing such an instrument to operate within a narrow bore tube. The size of the gyro rotor, mounted with its axis across the tube, is severely limited and makes satisfactory precession drift rates very difficult to attain in practice since gimbal bearing friction must be very low to compensate for the lack of gyrospin inertia. The usual problems associated with gimbal geometry are also encountered when this type of instrument is used.